(1) Field of the Invention
The present invention relates to a composition suitable for the preparation of a gasket for use in the production of shaped resin articles by cast poymerization.
(2) Description of the Related Art
Cast polymerization has been widely utilized for the preparation of shaped articles of methacrylic resins, styrene resins, and unsaturated polyester resins. According to this polymerization process, gaskets composed of a vinyl chloride resin are interposed between two confronting inorganic glass or metal sheets to form a casting mold, and a polymerizable compound is cast in this casting mold and polymerized.
In this process, a tube of soft polyvinyl chloride containing a plasticizer is often used for the gasket. In this case, low-molecular-weight compounds of additives contained in the gasket, such as a plasticizer and a stabilizer, are eluted by the polymerizable compound to contaminate the casting surfaces of the mold sheets. Accordingly, when the mold sheet peeled after completion of the polymerization is used again, it must be washed and cleaned, and therefore, a great deal of labor is required, resulting in an increase of the manufacturing cost.
As means for obviating this disadvantage, there have been proposed a method in which the surface of a gasket substrate is non-adhesively wrapped by a film so that the gasket is not eluted by the polymerizable compound (Japanese Examined Patent Publication No. 43-29,675), and a method in which a polyfunctional compound is coated and cured on the surface of the gasket substrate (Japanese Examined Patent Publication No. 57-4,658. In the former method, however, since the gasket substrate is wrapped by the film, the sealing property is not satisfactory and complete prevention of the elution of the additives is impossible. In the latter method, since the surface of the gasket is covered with a crosslinkable cured coating, elution of the additives can be prevented, but since the compound coated on the surface of the gasket is expensive and a curing equipment is necessary, the manufacturing cost is increased. Moreover, since the resin composition of the gasket substrate is different from the resin composition of the crosslinked cured coating on the surface, the gasket cannot be utilized as a part of the gasket-forming material when, after the polymerization, the gasket is cut and recovered and is again molded into a gasket.